1. Field of the Invention
This invention relates to a method of joining columbium to to nickel and nickel based alloys and, more specifically, to a method of performing such joining under low pressure and low temperature conditions.
2. Brief Description of the Prior Art
Columbium (Cb) is a desirable material for use in environments requiring high temperature regimes (i.e., about 2000 to about 2500.degree. F.), such as for use in thermal protection systems. However, a problem has existed in joining refractory metals to group VIII metals (iron, cobalt, nickel) because brittle intermetallic phases are formed. For example, the phases formed between columbium and nickel include Cb, CbNi, CbNi.sub.3 and Ni. All of these phases have different crystal structures and, hence, elastic stresses exist between the phases. Subsequent relaxation of these stresses results in interphase cracks. In addition, CbNi and CbNi.sub.3 are very hard brittle phases. In order to overcome the above noted problems, physical vapor deposited (P.V.D.) molybdenum and diffusion bonded palladium interlayers or P.V.D. molybdenum and P. V.D. tungsten interlayers have been used. Bonding was carried out for 1 to 60 minutes with a bonding pressure of 1.4 ksi in the temperature range 1651 to 2012.degree. F. The interlayers prevented the formation of intermetallic phases and ensured that only solid solutions formed in the bonding zone. Such prior art is discussed in A. G. Fesenko et al., "Svar Proiz", 1978, No. 12, pgs. 16 to 18, translated in "Welding Production", December, 1979, 25(12), pgs. 18 to 21.
Fesenko's bonding method used a two element interlayer. This was done to provide a series of neighboring solid solutions through avoiding the intermetallics that would be formed if the one element interlayer (Mo) was used to bond columbium to nickel.
Fesenko introduced the molybdenum and tungsten interlayers using the expensive process of electron beam evaporation and deposition. The palladium was joined to the nickel using diffusion bonding. An important point to make is that P.V.D. coatings have to be used in order to avoid the formation of brittle cracks that had occurred in bonding using interlayers of the same metals in the form of strips and foil. This limits the practical application of this method (Mo and W cannot be electroplated). Another practical drawback is that the bonding was carried out in the solid state, and hence intimate contact and 100 percent bonding would cause problems. A final problem with regard to the Mo-Pd system is that there exists a depressed solidus/liquidus point (2258.degree. F.) at 60 wt. percent Pd/40 wt. percent Ni if Pd dilution is not fully carried out, thereby causing local melting of the joined area to possibly occur at high service temperatures.
In bonding of columbium to nickel and nickel based alloys, the ideal interlayer material will (a) have a bonding temperature which does not exceed 2200.degree. F. and a remelt temperature which exceeds 2500.degree. F.; (b) provide continuous solid solutions throughout the bonded area; (c) produce no intermetallics or, if it does, the intermetallics will not be deleterious; (d) provide a thin interlayer material and a bonding cycle long enough for dilution to occur if the potential for intermetallics exists, resulting in solid solutions and the removal of the intermetallics from the bond region; (e) avoid galvanic couples; (f) have a coefficient of thermal expansion between that of Cb and Ni to reduce elastic stresses and cracking on thermal cycling; and (g) be a material which will not boil off under re-entry conditions.
Columbium is mutually soluble with several metals. The reactions with the nickel are very important as they indicate the compatibility of the interlayer metals surveyed, titanium being the best because (a) a large solid solubility range exists to accommodate titanium in nickel without the formation of intermetallics; (b) a low melting point eutectic exists between nickel and titanium (i.e., 1728` F.) to allow the use of low brazing temperatures with significant outdiffusion of the titanium into the bulk of the nickel which will increase the remelt temperature of the joint; and (c) titanium has a coefficient of thermal expansion between that of columbium and nickel.